1. Field of the Invention
The present invention relates to automated assembly systems employing linear drive mechanisms, and more particularly to a system for performing palletizing using a plurality of linear actuators having associated controllers, and to a program which facilitates entry of palletizing parameters to the controllers, for controlling action of the linear actuators according to preset palletizing conditions.
2. Background of the Invention
Linear actuators have been widely used in automated assembly procedures due to their inherent robustness, ease of variation in size and configuration, and the accuracy with which such actuators can be controlled to transport parts or tools to a precise location while following tightly controlled patterns of movement.
A known type of electric linear actuator, to which the principles of the present invention are applied, is shown in FIGS. 1 through 3.
The electric actuator, generally denoted at 50 in FIGS. 1 through 3, comprises an elongate frame 52 as a base, a pair of elongate side covers 54a, 54b mounted respectively on transversely opposite sides of the frame 52, a pair of end covers 56, 57 mounted respectively on longitudinally opposite ends of the frame 52, and an elongate top cover 58 engaging upper surfaces of the side covers 54a, 54b. 
On the frame 52, there are mounted a drive mechanism 60 fixed to one end of the frame 52 and supporting one end of a ball screw 62, a bearing block 64 fixed to the other end of the frame 52 and supporting the other end of the ball screw 62, and a table mechanism 66 linearly displaceable between the drive mechanism 60 and the bearing block 64 by the ball screw 62 upon rotation thereof. A pair of transversely spaced guide members 68a, 68b is fixed to an upper surface of the frame 52 for linearly guiding the table mechanism 66 when the table mechanism 66 is displaced by the ball screw 62. The bearing block 64 comprises a support block 22 mounted on the frame 52, and the other end of the ball screw 62 is rotatably supported in the support block 22 by a bearing 24.
The frame 52 has two transversely spaced grooves 70a, 70b of an identical T-shaped cross section which are defined in a lower surface thereof and extend parallel to each other in the longitudinal direction of the frame 52. The transversely opposite sides of the frame 52 have respective engaging grooves 72a, 72b defined therein and extending longitudinally therealong for attachment of the side covers 54a, 54b. The frame 52 also has longitudinal passageways 74a, 74b, 76a, 76b defined therein for accommodating electric wires or hydraulic fluid conduits. The frame 52 has a pair of attachment grooves 77a, 77b defined in an upper surface thereof near the passageways 76a, 76b for attachment of detectors such as automatic switches or the like. The attachment grooves 77a, 77b also serve as wiring grooves for accommodating leads connected to the detectors.
The side covers 54a, 54b have respective substantially L-shaped hooks 78a, 78b disposed on inner sides thereof for engaging in the respective engaging grooves 72a, 72b. The hooks 78a, 78b can be fitted into respective engaging grooves 72a, 72b when the side covers 54a, 54b are pressed obliquely downward against the transversely opposite sides, respectively, of the frame 52. To detach the side covers 54a, 54b from the frame 52, the side covers 54a, 54b are pulled upwardly away from the transversely opposite sides, respectively, of the frame 52. Therefore, the side covers 54a, 54b can easily be installed on and removed from the frame 52.
As shown in FIG. 2, the table mechanism 66 comprises a ball screw bushing 80 for converting rotary motion of the ball screw 62 into linear motion, a pair of table blocks 82a, 82b holding transversely opposite sides, respectively, of the ball screw bushing 80, and a pair of holders 84a, 84b having a channel-shaped cross section interposed between the table blocks 82a; 82b and the guide members 68a, 68b. As shown in FIG. 1, the table blocks 82a, 82b have holes 86 defined in their upper surfaces for coupling a member that is mounted on the table blocks 82a, 82b, and the table block 82b has a recess 88 defined in its upper surface for positioning a workpiece highly accurately.
In the case of multi-axis configurations, the table blocks 82a, 82b can support yet another linear actuator thereon, to enable the movement of a table block of the thus-supported actuator in both X and Y directions, for example.
As shown in FIG. 3, the drive mechanism 60 comprises a housing 106 mounted on the frame 52 and having a bearing 104 to which one end of the ball screw 62 is rotatably supported, a motor 108, and an encoder 112 covered by a cover 110. The housing 106 which supports the ball screw 62 serves as a motor body.
The motor 108 is fixed to the housing 106, and comprises a stator 116 composed of a plurality of separate stator cores 114 that are joined together, a plurality of coils 118 wound around the respective stator cores 114, and an annular permanent magnet 122 serving as a rotor which is fixedly mounted on a reduced-diameter end 62a of the ball screw 62 through a sleeve 120.
As shall be explained in greater detail in connection with the preferred embodiments of the invention, typically a linear actuator, such as the one described above, will be provided as only one actuator in a multi-axis system of actuators, which are connected together as a structural system, for enabling movement of an end effector (for example a gripping tool) in two or more dimensions, for picking up objects (parts) and delivering them to a desired destination, for example an assembly station.
In many assembly operations, a plurality of identical parts for assembly are arranged in a specified array inside a pallet, in an ordered pattern of rows and columns, and the parts are required to be delivered sequentially to an assembly station where the parts are assembled onto another component located in the assembly station. In such a situation, it is convenient to undertake a so called palletizing operation. More specifically, a gripping tool which is movable under the power of actuators is moved to a first position to pick up a first part in the pallet array, grip the part and deliver it to an assembly position where the part is assembled onto another, usually larger, component. After the part is assembled onto the component, the actuators are again driven to return the gripping tool to the pallet array, however, at this time the gripping member is controlled through a series of pre-stored movement patterns to be indexed to the next part in the array (for example, the part in the next column of the first row) so that the second part is then picked up and delivered to the assembly station for assembly onto the same or another component. When the first row of parts has been completed, the actuator can be indexed to the next row, proceeding in sequence until all parts in the pallet have been delivered to the assembly station.
Typically, movement of a linear actuator is controlled by inputting into a stored motion profile table (MPT) various parameters to control movement of the table blocks 82a, 82b or any member attached to the table blocks, at a set acceleration, velocity and deceleration, until reaching the desired position. A plurality of pre-programmed movements are each stored in the motion profile table as steps, wherein each step moves the table blocks 82a, 82b to one predetermined position according to a given acceleration-velocity-deceleration profile. When a series of movements is to be performed in sequence, a plurality of steps must first be pre-programmed and stored, and then the series of steps is run in a chosen order dictated by a Programmable Logic Controller (PLC). In multi-axis systems, of course, each actuator must be programmed to execute its own movements per each step in concert with the other actuators, adding more complexity.
In a palletizing operation, however, it is easily understood that because the parts in the pallet are arranged in an ordered array of rows and columns, the movements of the actuators for positioning a gripping tool to pick up the parts are highly regularized. That is, with each return of the gripping tool to the pallet to pick up a new part, it is simply indexed by one row or one column from the position at which the last part was previously picked up. Therefore, while it is conceivable to pre-program each return to the pallet as its own individual step, to do so is highly inefficient and fails to take advantage of the ordered arrangement of parts stored in the pallet.
Moreover, the motion profile table in a typical linear actuator controller allows for only about 128 individual steps to be set and pre-stored. However, a 20 rowxc3x9710 column pallet, which is not unusual, involves 200 separate part positions and will easily exceed the storage capacity for individual step moves in the motion profile table.
A palletizing program based on an understanding of the ordered structure of rows and columns for controlling a robotic arm, for example, has been disclosed in U.S. Pat. No. 4,969,109. However, this scheme does not provide a method for performing palletizing operations for a system of interconnected and mutually operable linear actuators. Further, the system lacks any kind of ergonomic interface enabling easy setting and adjustment of palletizing parameters by simply entering interval separations between objects arranged in the columns and rows of the pallet.
It is a principal object of the present invention to provide a system and method for controlling linear actuators to perform palletizing operations, which enables palletizing patterns to be entered as a set of simple pitch parameters corresponding to the intervals between rows and columns at which articles are arranged in a pallet, and then automatically generating a motion pattern of an end effector for picking up each article sequentially.
It is a further object of the present invention to provide a system and method for controlling a linear actuator wherein a palletizing operation can be entered as single step in a motion profile table, rather than a series of individually entered steps.
A still further object of the invention is to provide a system and method for performing palletizing moves by computing the moves in real time within the digital signal processor of a controller which controls a linear actuator.